The rising costs of hydrocarbon-based fuels and increasing concern about the environmental effects of carbon dioxide emissions have resulted in a growing demand for motor vehicles that operate either partly or entirely on electrical energy.
Hybrid Electric Vehicles (HEV) make use of both electrical energy stored in re-chargeable batteries and the mechanical energy converted from fuel, usually hydrocarbon based, by a conventional internal combustion engine (ICE). The batteries are charged during driving operation by the ICE and also by recovering kinetic energy during deceleration and braking. This process is offered by a number of vehicle original equipment manufacturers (OEMs) for some of their vehicle models. HEVs typically provide a normal driving experience, with the principle advantage of improved fuel consumption in comparison to conventional ICE only vehicles. Plug-in Hybrid Electric Vehicles (PHEVs) have similar functionality to HEVs, but in this application the battery can also be connected to the mains electrical system for recharging when the vehicle is parked. PHEVs typically have larger battery packs than HEV which affords some all-electric range capability. Dynamic driving will use electric power and ICE, though the area of operation using an internal combustion engine (ICE) for propulsion may be restricted to cruising and intercity driving. Consequently the fuel appetite of vehicles may well be different from that required currently for conventional ICE or HEV equipped vehicles. For vehicles based exclusively in an urban environment, the increased EV mode capacity and plug-in charging function further reduce the level of ICE activity. This can lead to significantly extended residence time for the fuel tank contents compared to HEV and conventional ICE vehicles.
Conventional ICE vehicles deliver about 600 km (400 miles) range for a propulsion system weight of about 200 kg and require a re-fill time of around 2 minutes. In comparison, it is considered that a battery pack based on current LiON technology that could offer comparable range and useful battery life would weigh about 1700 kg. The additional weight of the motor, power electronics and vehicle chassis would result in a much heavier vehicle than the conventional ICE equivalent.
In a conventional ICE vehicle, the engine torque and power delivery from the engine must cover the full range of vehicle operating dynamics. However, the thermodynamic efficiency of an internal combustion engine cannot be fully optimised across a wide range of operating conditions. The ICE has a relatively narrow dynamic range. Hence a major challenge for the vehicle manufacturers (OEMs) is to develop engine technologies and transmission systems that allow the engine torque and power delivery from the engine to operate over the full range of vehicle operating dynamics. Electrical machines on the other hand can be designed to have a very wide dynamic range, e.g., are able to deliver maximum torque at zero speed. This control flexibility is well recognised as a useful feature in industrial drive applications and offers potential in automotive applications. Within their operating envelope, electrical machines can be controlled using sophisticated electronics to give very smooth torque delivery, tailored to the demand requirements. However it may be possible to provide different torque delivery profiles that are more appealing to drivers. Hence this is likely to be an area of interest going forward for automotive designers. At higher speeds, electrical drive systems tend to be limited by the heat rejection capacity of the power electronics and the cooling system for the electric motor itself. Additional considerations for high torque motors at high speeds are associated with the mass of the rotating components, where very high centrifugal forces can be produced at high speeds. These can be destructive. In HEVs and PHEVs, the electric motor is therefore able to provide only some of the dynamic range. However, this can allow the efficiency of the ICE to be optimised over a narrower range of operation. This offers some advantages in terms of engine design.
Hence, current hydrocarbon fuels developed for a full range ICE may not be optimised or indeed beneficial for HEV or PHEV ICE units. Fuels have been formulated and regulated for conventional ICE vehicles for many years and may therefore be considered to have stabilised, with degrees of freedom in the formulation space well understood. The relatively recent introduction of hybrid technology presents an opportunity to consider the fuel formulation space from an entirely new perspective.